Edge coating of flat wires



May12, 1970 G. v. MARELL mL 3,511,680

' EDGE OATING oF FLAT WIRES Filed July 5. 1966 "0 7' cupi P/Paoacr En 6E-rsum-nass Ww fr l /5 /6 L PIE. 5

il 'l Mumia I 3 .7o/mf M fwvzzs United States Patent O 3,511,680 EDGECOATING OF FLAT WIRES Gerald V. Marcell and Harold I. Reynolds,Northfield,

Minn., and .lohn W. Andersen, Wheeling, W. Va., as-

signors to G. T. Schjeldahl Company, Northfield, Minn.,

a corporation of Minnesota Filed July 5, 1966, Ser. No. 562,558 Int. Cl.B44d 1/02 U.S. Cl. 117-43 3 Claims ABSTRACT OF THE DISCLOSURE Anapparatus and technique for the coating of the edge surfaces of flatribbon conductors with an adherent coating of a dielectric insulatingmaterial, which includes means for passing the ribbon conductors betweena pair of generally axially aligned rollers, the edge surfaces of theconductor being disposed adjacent to and generally tangentially to theconfronting surfaces of the roller so as to form a fillet of dielectricmaterial along the edge surface of the conductor.

The invention described herein was made in the performance of work undera NASA contract and is subject to the provisions of Section 305 of theNational Aeronautics and Space Act of 1958, Public Law 85-568 (72 Stat.435; 42 U.S.C. 2457).

The present invention relates generally to a technique for coating theedges of relatively thin substantially fiat ribbon conductors, and morespecifically to a technique for coating the edge surfaces of these thinribbon conductors with a film of a heat curable resinous substance,which can be cured to a substantially rigid form so as to establish auniform integral bond of a dielectric substance about the periphery ofthe ribbon conductor which is Vboth mechanically and electricallycontinuous and uniform.

In the past, various attempts have been made at coating the entireperiphery of various ribbon-like conductors, such as, for example, thosecertain ribbon-like conductors having a cross-sectional thicknessdimension which is less than about 8 mils. When conductors approach thisextremely thin cross-sectional dimension, they become exceedinglydiiiicult if not impossible to coat along the edge surfaces thereof.This is due in part to the inherent tendency of the surface tension ofthe coating substance per se to restrict its coating function to onlythe major surfaces of the ribbon-like conductor. Attempts at coating themajor surfaces of the thin ribbon-like conductors, rwhile simultaneouslyattempting to coat the edge surfaces thereof will normally not succeedbecause of the inherent tendency of the surface tension of the coatingmaterial to gather or accumulate this material along the broad surfacesof the conductor. This unfortunately results in ruptured coatings orextremely thin coatings and thereby leaves the edges generally exposedand accordingly does not provide the required electrical andenvironmental protection for the conductor along the lateral dimensionsthereof.

In accordance with the present invention, the edge surfaces of theconductor are coated with an adherent bond of dielectric 'by means of aroller or other surface which is itself being continuously provided witha film or other coating of material for transfer to the edge surfaces ofthe conductor. This edge-coating operation may occur either prior to orsubsequent to the coating of the dat surfaces of the conductor.Generally speaking, it is preferred that the coating be cured at leastthrough the B- stage or further, in order to enhance the over-allultimate cure of the dielectric coating substances. Furthermore, it isgenerally preferred that multi-passes be employed with "ice the coatingmaterial in order that a strong, complete and integral film be availableto the conductor. After the individual ribbon conductors have beenencapsulated, the encapsulation including the coating of the flatsurfaces and the edge surfaces, the ultimate integral coating is finallycured and dried, and the conductor as coated is then ready for itsultimate disposition or use. If desired, one of the iinal coatings alongthe edges may be a conductive coating prepared in a manner similar tothe dielectric coating using, for example, dispersion of a iinelydivided metal such as copper, silver or the like. Such an arrangementmay iind application as a ymulti-conductor unit such as, for example, acombination word-sense line-inhibit line for use in data processingapplications.

The present invention linds application in the preparation ofhigh-density flexible cables, which iiexible cables normally employ aplurality of spaced apart iiat ribbonlike conductors, these conductorsnormally being spaced substantially equally, from edge-to-edge, acrossthe width of the iiexible cable assembly. In order to create ahighdensity flexible cable of this type which is capable of handlinghigh potential differences between adjacent conductors, the edgesurfaces of the conductors must be effectively coated with a substantialdielectric insulation layer integrally bonded to the surface of theconductor.

Therefore, it is an object of the present invention t0 provide animproved technique for coating the edge surfaces of at ribbon-likeconductors with a coating of a dielectric insulation.

It is yet a Ifurther object of the present invention to provide animproved technique for the application of a dielectric coating to theedge surfaces of a flat ribbon-like conductor having a cross-sectionalthickness dimension of less than about 8 mils.

It is still a further object of the present invention to provide animproved technique for applying a uniform integral coating of adielectric insulation to the edge surface of a iiat ribbon-likeconductor, the coating being applied in heat-curable form to these edgesurfaces, and capable of being thereafter cured to a substantialinsulating layer of proper and desirable mechanical and electricalproperties.

It is yet a further object of the present invention to provide animproved technique for applying a coating of a dielectric insulationmaterial to the edge surfaces of flat ribbon-like conductors wherein thecoating is applied as a viscous heat-curable substance to these edgesurfaces, the application being made by means of a pair of oppositelydisposed axially aligned spaced apart rollers with the liat ribbon-likeconductor disposed and moving substantially tangentially relative to thespaced apart confronting surfaces of the axially aligned rollers.

Other and further objects of the present invention will become apparentto those skilled in the art upon a study of the following specification,appended claims, and accompanying drawing wherein:

FIG. 1 is a perspective View 0f a system arranged for coating the edgesurfaces of a flat ribbon conductor in accordance with the technique ofthe present invention;

FIG. 2 is a detail end view of a pair of edge coating rollers disposedin generally axially aligned relationship with a iiat ribbon conductorrunning generally and tangentially to the confronting surfaces of therollers;

FIG. 3 is a detail perspective view of a cross-section of a conductorhaving an adherent dielectric insulation iilm bonded to the surfacesthereof;

FIG. 4 is a view similar to FIG. 3, on a slightly smaller scale, showingan assembly of these at ribbon conductors in a high-density pattern; and

FIG. 5 is a schematic diagram showing the various steps which may becarried out in the preparation of a coated ilat ribbon conductor productin accordance with the present invention.

In accordance with the preferred modification of the present invention,and particularly as illustrated in FIG. 1 of the drawings, the systemshown generally at 10 provides an arrangement for coating the edgesurfaces of the flat ribbon-like conductor 11, and further provides anarrangement for drawing the coated conductor through treating zones andonto take-up rolls subsequent to the application of the coating materialto the edge surfaces thereof. Generally, the at ribbon conductor 11 ispassed through the area between the confronting surfaces of the rollers12 and 13, such as is shown in detail in FIG. 2, the surface of therollers 12 and 13 being continuously provided with a lm of a heatcurable dielectric insulation material, such as is shown in the fillets15 and 16. This material may be applied to the surfaces of the rollers12 and 13 by any conventional means, such as, for example, by means of apair of hoppers 18 and 19 which are provided with doctor blades or thelike for controlling the thickness of the lm on the surface of therollers. The system provides a technique for applying a coating to theedges of the flat ribbon conductors in a uniform manner which coatingwill resist rupture or breaking away, even when the conductors are lessthan about 8 mils in thickness.

After receiving the coating on the edge surfaces, the conductor ispassed through a heating zone 20, this heating zone 20 including aplurality of idler rolls 21-21 which are journaled for free rotation,and which accordingly provide a multitude of flights, runs or passeswithin the chamber 20. It will be appreciated that the time-temperaturecure relationship for the dielectric insulation being employed willdetermine the conditions within the environment of the chamber 20, andthose skilled in the art can readily adapt the requirements of theparameters of the curing cycle to the needs and requirements of thechamber 20. The single criteria fulfilled is that the material remainwithin the connes of the chamber 20 for a period sufficiently long toprovide a substantially complete cure of the material coated onto theedges f the flat ribbon-like conductor. After leaving the chamber 20,the flat ribbon-like conductor 11 is taken up on a take-up spool 23,this spool being journaled for axial rotation about the central shaft24.

It will be observed that this operation may take place on a roll-to-rollbasis, for efliciency in operation. Also, it will be appreciated that aplurality of stages may be ernployed in the system whereby the flatsurfaces are initially coated, and the ribbon then moved through achamber such as the chamber 20 for curing the material on the surface ofthe conductor. In the event that flat surfaces are being coated and thematerial subsequently cured thereon, it will normally be desirable toprovide a reasonably long initial pass within a heating chamber such asthe chamber 20 to accomplish substantial or significant cure of thematerial prior to its being passed directly over and upon a rollersurface.

With particular attention being directed to FIG. 3, it will be seen thatthe conductor 11 is provided with an adherent film such as a dielectricinsulation film along the flat surfaces such as is shown at 26 and 27,together with an edge coating of such an adherent film such as is shownat 28 and 29. The over-all cross-sectional conguration can be stated tobe in the form of a dogbone, with the edge surfaces being built up to anextent greater than the composite or aggregate thickness of the layers26 and 27 and the conductor 11. With reference to FIG. 4 of thedrawings, it can be observed that the individual conductor elements 11together with the dielectric insulation coatings applied thereto, vareencapsulated between layers of an insulating substance such as a film ofa relatively flexible material such as polyethylene terephthalate orother polyester substances, polyimideamide materials, epoxy basedsubstances and the like.

These ilm forming materials are of course, generally commerciallyavailable.

In FIG. 5, there is shown a schematic illustration of the operationswhich may be employed in carrying out the various aspects of the presentinvention. Generally, the starting point is selected as the coating ofthe fiat surfaces, such as in the upper lefthand corner of FIG. 5, thisbeing followed by a curing and drying operation, which operations may berepeated, if necessary to achieve the appropriate thickness of coatingon the major surfaces of the flat ribbon conductor. Subsequent to the atsurface coating operation, the edge surfaces are coated and this edgesurface coating cured and dried. This coating of the edge surfaces maybe repeated a number of times in order to achieve the proper thicknessdesired on the product. If desired, a liinal flat surface coating may beapplied, this being followed, if desired, by an additional coating ofthe edge surfaces after which the system is cured and dried, and theproduct obtained therefrom. It will be appreciated that these variousoperations may be established on a continuous basis between individualstations arranged in spaced relationship between a supply roll such asis shown at 17, and the take-up roll such as is shown at 23. Asindicated, previously, a coating of a conductive film may be applied tothe surface of the dielectric film to form a multiconductor system.

As a raw material for the dielectric insulation coating, apolyimide-amide solution may be employed, this solution normally beingthickened by the addition of a finely divided powder thereto, such as,for example, a silicate such as a quantity of powdered silica, or thelike. Powdered silicon dioxide is available commercially under the nameCab-O-Sil or Aerosil A hydrophobic material may -be preferred as apowder ller for many systems. Polyimide-amide Ifilm forming solutionsare available commercially. It has been found that the relativelyviscous solution proper for coating thin conductors may contain from 3-5percent SOZ, with a dielectric base material of polymide-amide in asolvent such as DMSO or the like, the polyimide-amide solutionconstituting 15-18 percent of the mixture, balance solvent. Theresultant material will have the proper viscosity for coating ribbonlikeconductors in accordance with the technique of the present invention. Ithas been found that this composition becomes too viscous when more thanabout 6` percent of S102 is added to the mixture. A similar amount ofmagnesium silicate has been found to be appropriate for the coatingcomposition. The curing cycle will generally constitute a technique fordriving the solvent from the iilm and thereafter curing the polymermaterial. The individual curing cycles are preferably interrupted at theBstage in order to render the coating somewhat more flexible and therebyless likely to crack under the curing process.

Generally, a coating of about 0 1 to about 0.2 mil thick (after curing)can be deposited per coating pass. Preferably, this is built up to about1 mil thickness after the total number of passes has been completed.When applied in accordance with the present invention, this coating isgenerally uniform about the surface of the conductor and will minimizeany oxidation reaction along the surface of the conductor.

The illustration in FIGS. 1 and 2 shows the -wre proceeding in the samedirection as the mating or confronting surfaces of the individualrollers 12 and 13. It will be appreciated that a technique may beutilized wherein the roller surfaces move counter to the direction ofmotion of the conductor being coated. The provision of a fresh film ofcurable material on the surface of the roller is thus believed tocontribute to the coating capability of these thin edge surfaces.

The coating of the major surfaces of the fiat conductor may preferablybe accomplished by utilizing a pair of rollers as shown for the edgecoating concept, or as an alternative, a technique employing felt padsdisposed in contact with the major surfaces of the flat ribbon conductormay be employed. At any rate, the techniques employed for coating themajor surfaces are not deemed critical to the operation, since variousknown techniques may be suitably employed.

In order to remove a substantial quantity of the solvent employed in theraw dielectric coating solution substance, the coating, upon leaving thecoating zone or station may be exposed to a flow or blast of heated air.This technique will achieve a substantial solvent removal at the timethat the conductor is leaving the coating station.

For best coating techniques, it is generally preferred that theconductor, for example copper be reasonably free of adherent oxide orsulde films. Accordingly, reasonable cleaning and deoxidizing steps maybe employed immediately prior to the initiation of the coatingoperation. Until the coating film covers substantially the entiresurface of the conductor, the exposure to high temperatures andatmospheres should be avoided. Generally speaking, a curing cycle ofless than about 400 F. should be employed prior to the time the entiresurface of the conductor is coated. Subsequently, during the variousadditional coating operations, curing temperatures of up to about 700 F.may be encountered Without risking damage to the surface of theconductor ribbon. If desired, a deoxidizing operation may be added on anin-line basis to the remaining steps in the sequence.

What is claimed is:

1. The method of coating the edge surfaces of at ribbon conductors withan adherent coating of a dielectric insulating material comprising:

(a) coating the flat surfaces of said ribbon conductor with an adherentcoating of a dielectric insulating material;

('b) curing said coating of dielectric insulating material to agenerally hardened state;

(c) maintaining a generally uniform raw lm of a coating along theconfronting surfaces of a pair of generally convex members with a raw lmof a dielectric insulating material;

(d) drawing the fiat ribbon conductor through a gap zone formed by saidgenerally convex surfaces with the edge surfaces of said conductor beingdisposed in contact with said raw film and generally tangentially to theconfronting convex surfaces; and

(e) curing said edge coating of dielectric insulating material to agenerally hardened state.

2. The edge coating method as defined in claim 1, being particularlycharacterized in that said convex members are rollers rotating in aperipheral direction of movement which substantially matches thedirection and rate of movement of the flat ribbon conductor through saidgap zone.

3. The edge coating method as dened in claim 1, being particularlycharacterized in that means are provided for thermally curing saidcurable resin after each coating operation.

References Cited UNITED STATES PATENTS 1,908,142 5/ 1933 Gregg et al117--43 X 2,058,454 10/ 1936 Hopkins.

2,566,151 8/1951 Wright 117-43 X 3,218,187 11/1965 Wade 117-43 ALFRED L.LEAVITT, Primary Examiner C. R. WILSON, Assistant Examiner U.S. Cl. X.R.117-111

